Shock Effects on Magnetic Remanence in Rocks from the Dhala Impact Structure, India

Impact-generated shock waves can modify the remanence properties of magnetic minerals in target rocks, but their effects remain poorly understood. This study examines shock-induced modifications in the magnetic properties of both unshocked lithologies and impactites at the Dhala impact structure in India. The structure formed during the Paleoproterozoic (2.5–1.7 Ga) and lies within the Bundelkhand craton of the Indian Shield, with an original diameter of ~11 km. The unshocked target lithologies are primarily granitoid, while two types of impactites, impact melt rock and monomict breccia, are prominently exposed at the surface. Primary magnetic carriers are multidomain (MD) Ti-poor magnetite in unshocked rocks; pseudo–single-domain (PSD) Ti-magnetite with minor Ti-hematite and pyrrhotite in impact melt rock; and pseudo–single-domain to single-domain (PSD–SD) Ti-magnetite with minor Ti-hematite in monomict breccia (Pandey et al., 2026). To evaluate coercivity spectra, domain state, and remanence efficiency, alternating field (AF) demagnetization, isothermal remanent magnetization (IRM), and anhysteretic remanent magnetization (ARM) experiments were conducted on unshocked granitoid rocks and on impactites.

AF demagnetization results reveal distinct magnetic decay patterns among the lithologies. Unshocked rocks exhibit a peak in magnetization intensity at 10 mT, likely due to removal of a low-coercivity antiparallel component, followed by gradual decay, with median destructive field (MDF) values of ~20–60 mT and 65–90% loss of magnetization at 100 mT. Impact melt rocks exhibit smoother decay trends, with MDFs ranging from 10 to 40 mT, corresponding to 75–98% loss of magnetization at 100 mT. In contrast, monomict breccia displays the most unstable behavior with fluctuating magnetization intensity, often retaining >50% of remanence at 100 mT. The average mass-normalized saturation IRM_1000mT values vary systematically among lithologies, with the lowest (4.18×10^-3 Am² kg^-1) in monomict breccia, intermediate (1.11×10^-2 Am² kg^-1) in impact melt rocks, and the highest (6.7×10^-1 Am² kg^-1) in unshocked rocks. Average mass-normalized ARM values follow a similar trend, with the lowest (2.18×10^-6 Am² kg^-1) in monomict breccia, intermediate (3.65×10^-5 Am² kg^-1) in impact melt rocks, and the highest (1.03×10^-2 Am² kg^-1) in unshocked rocks.

Together, these results demonstrate a progressive reduction in remanence acquisition capacity and magnetic stability from unshocked rocks to monomict breccia. Overall, the findings highlight that impact-generated shock waves significantly modify the domain state, coercivity spectrum, and remanence efficiency of the target rocks.

Reference: Pandey, A. K., Agarwal, A., Joshi, G., Sangode, S., & Venkateshwarlu, M. (2026). Shock demagnetization in an ambient magnetic field at the Dhala impact structure, India. Communications Earth & Environment. https://doi.org/10.1038/s43247-025-03164-6